Antifouling coating, heat exchanger provided with same, and method for manufacturing heat exchanger

ABSTRACT

The present invention provides an antifouling coating formed from a water-based coating composition comprising 0.1% by mass to 10% by mass of ultrafine silica particles having an average particle size equal to or less than 25 nm, 5% by mass to 50% by mass, relative to the ultrafine silica particles, of a zirconium compound which is at least one selected from zirconium chloride and zirconyl chloride, and 30% by mass to 99.5% by mass of water. In accordance with the present invention, it is possible to provide an antifouling coating that can maintain the antifouling performance and hydrophilicity and prevent corrosion of fins even under an environment with a large amount of contaminating substances, such as metal particles, in the air.

TECHNICAL FIELD

The present invention relates to an antifouling coating that can preventadhesion of contamination to a surface, and to a heat exchanger providedwith the coating. More particularly, the present invention relates to anantifouling coating for use in metal processing (welding, cutting, etc.)sites, enterprises where metal dust is handled, vehicles such as railwayvehicles, and related facilities, and also to heat exchangers for airconditioners.

BACKGROUND ART

Surfaces of various products can look dirty, cause sanitary problems, ordemonstrate performance degradation caused by corrosion or the like, dueto exposure to various contaminating substances from the environment.Among such products, heat exchangers in air conditioning equipment arehighly susceptible to contamination from the environment due to thefunctions thereof, and the contamination easily causes a variety offailures.

A heat exchanger has a structure in which a large number of fins (forexample, aluminum fins) are attached to a pipe in which a coolant flows,and heat exchange efficiency is increased by fins with a large surfacearea. Condensed water can easily adhere to the fin surface duringcooling and warming, and the ventilation resistance can increase andheat exchange efficiency can decrease due to a phenomenon of the finsbeing connected to each other by the condensed water (referred tohereinbelow as “bridge phenomenon”). In particular, since the bridgephenomenon is easily induced by contamination such as dust adhering tothe fin surface, the bridge phenomenon is typically prevented by formingan organic or inorganic hydrophilic coating that excels in antifoulingperformance on the fin surface. In the present description, the“antifouling performance” means the performance such that contaminationis unlikely to adhere and the adhered contamination is easily removed.

However, when an inorganic hydrophilic coating contains a large amountof an inorganic component such as water glass or boehmite, the coatingeasily adsorbs odor. For this reason, organic hydrophilic coatings areoften used. Meanwhile, organic components of organic hydrophiliccoatings are easily decomposed, degraded, or dissolved in condensedwater with the passage of time, and the antifouling performance orhydrophilicity of the hydrophilic coating are degraded.

Accordingly, techniques for forming hydrophilic coatings by usingvarious coating materials have been suggested for preventing thedegradation of antifouling performance and hydrophilicity of hydrophiliccoatings. For example, Patent Document 1 suggests a method for forming ahydrophilic coating by using a coating material comprising a modifiedpolyvinyl alcohol and a crosslinking agent. Patent Document 2 suggests amethod for forming a hydrophilic coating by using a coating materialcomprising carboxymethyl cellulose, polyethylene glycol, and acrosslinking agent. Patent Document 3 suggests a technique for forming athin film constituted by ultrafine silica particles and fluororesinparticles. By using a configuration in which hydrophobic portions arepresent in a spot-like manner on a hydrophilic surface, it is possibleto prevent the adhesion of hydrophilic or hydrophobic contaminants ofvarious properties, while maintaining high hydrophilicity of the entirefilm. Since the film is thin, the problem of odor adsorption can beavoided and the film can be adapted to a heat exchanger.

PRIOR ART REFERENCES Patent Documents

Patent Document 1: Japanese Patent Application laid-open No. 10-36757

Patent Document 2: Japanese Patent Application laid-open No. 8-261688

Patent Document 3: WO 2008/087877

SUMMARY OF INVENTION Technical Problem

The antifouling performance and hydrophilicity of an antifouling coatingare typically degraded by exposure to water, heat, sunlight, and air,and the degradation of coating can result in metal corrosion or resindegradation in the fin on which the coating is provided. In particular,when a contaminating substance includes a large amount of metalparticles of iron, copper, or alloys thereof, more particularly an ironpowder, it can easily cause the degradation of antifouling performanceand hydrophilicity of the hydrophilic coating and corrosion of fin.Patent Documents 1, 2, and 3 do not resolve the problem relating todegradation of antifouling performance and hydrophilicity of hydrophiliccoating and corrosion of fin which is caused by adhesion of suchcontaminating substances.

Thus, the hydrophilic coatings described in Patent Documents 1, 2, and 3can maintain the antifouling performance and hydrophilicity under theusual environment with a small amount of contaminating substances, suchas metal particles, in the air, but the antifouling performance andhydrophilicity gradually degrade due to the adhesion of thecontaminating substances under an environment with a large amount ofcontaminating substances such as metal particles (for example, in metalprocessing (welding, cutting, etc.) sites, enterprises where metal dustis handled, vehicles such as railway vehicles, and related facilities).More specifically, since the hydrophilic coatings described in PatentDocuments 1 and 2 are organic coatings, metal ion components created bydissolution of the contaminating substances in condensed water act as acatalyst and enhance the degradation of organic components. As a result,the antifouling performance and hydrophilicity of the hydrophiliccoatings are degraded and fins are corroded. In Patent Document 3, as aresult of corrosion of even a very small number of metal particlesadhered to the thin film, metal ions that have permeated through thefilm degrade the base material, or the antifouling performance andhydrophilicity can be degraded by corroded iron particles firmly bondedto the surface of ultrafine silica particles.

The present invention has been created to resolve the above-describedproblems, and it is an objective thereof to provide an antifoulingcoating that can maintain the antifouling performance and hydrophilicityand prevent corrosion of fins even under an environment with a largeamount of contaminating substances, such as metal particles, in the air,and also provide a heat exchanger having such a coating, and amanufacturing method therefor.

Solution to the Problem

The inventors have conducted a comprehensive study with the object ofresolving the above-described problem, and the results obtaineddemonstrate that an antifouling coating formed from a water-basedcoating composition comprising ultrafine silica particles having aspecific average particle size and a zirconium compound which is atleast one selected from zirconium chloride and zirconyl chloride at aspecific ratio can maintain the antifouling performance andhydrophilicity even under an environment with a large amount ofcontaminating substances such as metal particles. This finding led tothe creation of the present invention.

Thus, the present invention provides an antifouling coating formed froma water-based coating composition comprising 0.1% by mass to 10% by massof ultrafine silica particles having an average particle size equal toor less than 15 nm, 5% by mass to 50% by mass, relative to the ultrafinesilica particles, of a zirconium compound which is at least one selectedfrom zirconium chloride and zirconyl chloride, and 30% by mass to 99.5%by mass of water.

The present invention also provides a heat exchanger comprising a pipein which a coolant flows, and a fin attached to the pipe, the heatexchanger comprising a hydrophilic organic coating formed on the fin, areaction layer obtained by reacting a zirconium compound which is atleast one selected from zirconium chloride and zirconyl chloride on asurface layer of the hydrophilic organic coating, and an inorganiccoating provided on the reaction layer and formed from a water-basedcoating composition comprising 0.1% by mass to 10% by mass of ultrafinesilica particles having an average particle size equal to or less than15 nm.

The present invention also provides a heat exchanger comprising a pipein which a coolant flows, and a fin attached to the pipe, the heatexchanger comprising a hydrophilic organic coating formed on the fin,and an inorganic coating provided on the hydrophilic organic coating andformed from a water-based coating composition comprising 0.1% by mass to10% by mass of ultrafine silica particles having an average particlesize equal to or less than 15 nm, 5% by mass to 50% by mass, relative tothe ultrafine silica particles, of a zirconium compound which is atleast one selected from zirconium chloride and zirconyl chloride, and30% by mass to 99.5% by mass of water.

Advantageous Effects of the Invention

In accordance with the present invention, it is possible to form anantifouling coating which has high hydrophilicity and antifoulingability and resists to adhesion of contaminants and degradation even inthe presence of metal particles or the like. Further, in a heatexchanger provided with a fin with the antifouling coating formedthereon, antifouling performance and hydrophilicity can be maintainedand the fin can be prevented from corrosion even under an environmentwith a large amount of contaminating substances, such as metalparticles, in the air.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the degradation of theconventional coating caused by adhesion of metal particles.

FIG. 2 is a schematic diagram demonstrating that the antifouling coatingaccording to Embodiment 1 of the present invention is unlikely to bedegraded by the adhesion of metal particles.

FIG. 3 is a schematic diagram illustrating the degradation of theconventional coating caused by adhesion of metal particles.

FIG. 4 is a schematic diagram demonstrating that the antifouling coatingaccording to Embodiment 2 of the present invention is unlikely to bedegraded by the adhesion of metal particles.

DESCRIPTION OF EMBODIMENTS Embodiment 1

At metal processing (welding, cutting, etc.) sites, enterprises wheremetal dust is handled, vehicles such as railway vehicles, and relatedfacilities, metal particles such as particles of iron, copper, zinc, oralloys thereof, and particles to which such metal particles have adheredfloat in the air (such particles will be together referred tohereinbelow as “metal particles”). A coating constituted by an inorganicmaterial, an organic material, or a combination thereof, in particularhighly hydrophilic coating having a large number of polar groups, suchas hydroxyl groups, on the surface, is degraded by adhesion of the metalparticles, or the metal particles are easily firmly attached thereto.

FIG. 1 is a schematic diagram illustrating the degradation of theconventional coating caused by adhesion of metal particles. In theconventional coating depicted in FIG. 1, ultrafine silica particles 11serving as fine inorganic particles are aggregated and fluororesinparticles 12 are present in a spot-like fashion. With the coating ofsuch a configuration, dust is unlikely to adhere, as will be describedhereinbelow, but when the coating is wetted with water, a metal particle13 (for example, an iron particle) adheres thereto. The metal particle13 is partially corroded by water, thereby forming metal ions 14adsorbed by the ultrafine silica particles 11 in the coating. Under suchconditions, the metal particle 13 is strongly attached to the coating,the coating is stained, and the antifouling ability is degraded by theattached metal particles 13.

Meanwhile, the water-based coating composition for obtaining theantifouling coating according to Embodiment 1 of the present inventioncomprises ultrafine silica particles 11, a zirconium compound which isat least one selected from zirconium chloride and zirconyl chloride, andwater. As a result of coating the water-based coating composition on anarticle surface and drying, it is possible to form an antifoulingcoating that can demonstrate high durability and antifouling abilitywith respect to the attachment of metal particles which is caused bycorrosion and reaction induced by the metal ions 14 and can demonstrategood hydrophilicity and antifouling ability even under an environment inwhich the metal particle 13 can easily adhere. FIG. 2 is a schematicdiagram demonstrating that the antifouling coating 15 according toEmbodiment 1 is unlikely to be degraded by the adhesion of the metalparticle 13. In FIG. 2, a Zr atom 16 introduced by zirconium chloride orzirconyl chloride forms a bond with a hydroxyl group, or the like, onthe surface of the ultrafine silica particles 11, thereby protecting thesurface of the antifouling coating 15. As a result of the introduced Zratom 16 acting as a protective group of the hydroxyl group present onthe surface of the antifouling coating 15, it is apparently possible toinhibit the adsorption of the metal ions 14, such as an iron ionsgenerated from the metal particle 13, and the attachment of the metalparticle 13. FIG. 2 illustrates a state in which the hydrophobicfluororesin particles 12 are present in a spot-like fashion in the thinhydrophilic silica film, but the fluororesin particles 12 furtherinhibit the adhesion of dust, and even when such particles are notpresent, the effect of inhibiting the attachment of the metal particle13 can be obtained as a result of adding zirconium chloride or zirconylchloride. Thus, since the surface of such antifouling coating 15 isconstituted by a continuous hydrophilic portion in which hydrophobicportions are present, when water droplets adhere thereto, the dropletsare easily spread and the film, when viewed as a whole, demonstrateshigh hydrophilicity. Further, because of a configuration in whichhydrophilic portions and hydrophobic portions are finely mixed, evenwhen hydrophilic dust or hydrophobic dust attempts to adhere, the dustcomes into contact with the surface having an opposite nature and cannotstably adhere to the surface. Thus, a strong antifouling ability isdemonstrated with respect to a wide range of dust-like contaminants.

There are various types of organometallic compounds acting similarly tozirconium chloride and zirconyl chloride, examples thereof includingalkoxides or chelates other than chlorides, and compounds of metalsother than zirconium, such as titanium and aluminum. However, bycontrast with those compounds, where zirconium chloride and zirconylchloride are added to a water-based coating composition comprising wateras the main component, they can be stably present therein as zirconiumhydroxide (zirconium chloride becomes zirconyl chloride in an aqueoussolution), high reactivity can be demonstrated when the coating isformed, and the effect of the metal particle 13 can be efficientlysuppressed. Further, where a zirconium compound which is at least oneselected from zirconium chloride and zirconyl chloride is added to awater-based coating compound, although hydrochloric acid, or the like,is produced as a byproduct, it is unlikely to remain in the antifoulingcoating 15 or, even when the byproduct remains, it produces littleeffect on antifouling ability. Another merit of zirconium chloride andzirconyl chloride is that it is very safe for humans. When otherorganometallic compounds are added, various byproducts derived fromchelates or alkoxides can be generated and can adversely affect theantifouling performance.

The amount of the zirconium compound which is at least one selected fromzirconium chloride and zirconyl chloride is 5% by mass or more to 50% bymass or less, preferably 10% by mass or more to 40% by mass or less onthe basis of the solid weight of the ultrafine silica particles. Wherethe content of the zirconium compound is less than 5% by mass, theeffect of the metal particle 13 cannot be sufficiently suppressed.Meanwhile, it is undesirable that the content of the zirconium compoundbe above 50% by mass because the pores in the porous silica film formedby cohesion of the ultrafine silica particles 11 are filled by thezirconium compound and, therefore, the antifouling coating 15 becomestoo dense and the antifouling performance is degraded. The content ofthe zirconium compound, as referred to herein, is calculated on thebasis of the mass of zirconium oxide when the entire zirconium in theadded zirconium compound becomes zirconium oxide (ZrO₂).

Colloidal silica and fumed silica can be used as the ultrafine silicaparticles 11 in accordance with the present invention. The averageparticle size of the ultrafine silica particles 11 may be equal to orless than 25 nm, preferably 3 nm or more to 25 nm or less, morepreferably 5 nm or more to 15 nm or less. The “average particle size” ofthe ultrafine silica particles 11 in the present description means thevalue of the number-average particle size of the ultrafine silicaparticles 11 measured by a particle size distribution meter of a laserbeam scattering system or dynamic optical scattering system. Where theaverage particle size of the ultrafine silica particles 11 is too small,the particles are sometimes difficult to disperse uniformly in thewater-based coating composition. Further, the dust adhesion inhibitioneffect demonstrated by the antifouling coating 15 is sometimes difficultto obtain. Meanwhile, where the average particle size of the ultrafinesilica particles 11 exceeds 25 nm, the strength of the film of theantifouling coating 15 decreases and durability suitable for practicaluse sometimes cannot be obtained. The dust adhesion inhibition effectdemonstrated by the antifouling coating 15 is obtained because theantifouling coating 15 is a porous silica film formed by cohesion of theultrafine silica particles 11. This result can be explained as follows.As a result of the antifouling coating 15 being a porous silica film,even when dust collides with the surface, the contact surface areathereof is extremely small, and since the film density is low, anintermolecular force acting between the dust and the film is small.Where the condition of the average particle size being equal to or lessthan 25 nm is fulfilled, the ultrafine silica particles 11 may contain acertain amount of particles with an average size above 25 nm. Forexample, by using the ultrafine silica particles 11 with a twin-peakparticle size distribution having particle size distribution peakswithin a range from 5 nm or more to 15 nm or less and a range from 30 nmor more to 120 nm or less, it is possible to increase adequately thedepressions and protrusions on the surface of the antifouling coatingand improve the antifouling ability.

When a water-based coating composition is prepared, it is preferred thata dispersion of the ultrafine silica particles 11 that has a pH equal toor less than 4 be used in order to prevent cohesion when the zirconiumcompound which is at least one selected from zirconium chloride andzirconyl chloride is added. Further, in order to facilitate theformation of the antifouling coating 15, a typical binder such as asilicate, for example, sodium silicate and lithium silicate, a metalalkylate, a phosphoric acid amine, and ρ-alumina be used together withthe ultrafine silica particles 11.

The content of the ultrafine silica particles 11 may be 0.1% by mass ormore to 10% by mass or less, preferably 0.2% by mass or more to 10% bymass or less, more preferably 0.2% by mass or more to 4% by mass or lesswith respect to the water-based coating composition. Where the contentof the ultrafine silica particles 11 is less than 0.1% by mass, thethickness of the antifouling coating 15 is too small and a sufficienthydrophilicity and dust adhesion inhibiting effect cannot be obtained.Where the content of the ultrafine silica particles 11 is above 10% bymass, protrusions and depressions on the surface of the antifoulingcoating 15 formed become large, dust is easily caught thereon, andantifouling ability is degraded.

The type of the fluororesin particles 12 is not particularly limited andsuitable particles can be formed, for example, from PTFE(polytetrafluoroethylene), FEP (tetrafluoroethylene-hexafluoropropylenecopolymer), PFA (tetrafluoroethylene-perfluoroalkyl vinyl ethercopolymer), ETFE (ethylene-tetrafluoroethylene copolymer), ECTFE(ethylene-chlorotrifluoroethylene copolymer), PVDF (polyvinylidenefluoride), PCTFE (polychlorotrifluoroethylene), PVF (polyvinylfluoride), fluoroethylene-vinyl ether copolymer, fluoroethylene-vinylester copolymer, copolymers and mixtures thereof, and compositionsobtained by mixing other resins with those fluororesins. The averageparticle size of the fluororesin particles 12 is preferably 50 nm ormore to 500 nm or less, more preferably 70 nm or more to 350 nm or less,most preferably 90 nm or more to 300 nm or less. The “average particlesize” of the fluororesin particles 12 in the present description meansthe value of the number-average particle size of primary particles ofthe fluororesin particles 12 measured by a particle size distributionmeter of a laser beam scattering system or dynamic optical scatteringsystem. Where the average particle size of the fluororesin particles 12is less than 50 nm, the effect demonstrated by the addition of thefluororesin sometimes cannot be demonstrated. Meanwhile, where theaverage particle size of the fluororesin particles 12 is above 500 nm,the protrusions and depressions on the surface of the antifoulingcoating 15 become too large and the antifouling effect can be lost.

When the fluororesin particles 12 are included in the water-basedcoating composition, the content thereof, as a mass ratio of theultrafine silica particles 11 and the fluororesin particles 12, ispreferably 70:30 to 95:5, more preferably 75:25 to 90:10. Where theratio of the fluororesin particles 12 is above the abovementioned range,the hydrophilicity of the antifouling coating 15 can become too low.Furthermore, a large number of hydrophobic portions caused by thefluororesin particles 12 are exposed on the surface, and lipophiliccontaminants can easily adhere. Meanwhile, where the ratio of thefluororesin particles 12 is much smaller than the abovementioned range,the hydrophobic portions caused by the fluororesin particles 12 are notsufficiently exposed on the surface of the antifouling coating 15,hydrophilic contaminants can easily adhere, and the desired dustadhesion inhibiting performance sometimes cannot be obtained. The massof the ultrafine silica particles 11 and the fluororesin particles 12herein is a value measured by drying the water-based coating compositionat 120° C. and removing the moisture.

The thickness of the antifouling composition 15 is not particularlylimited but is preferably 0.05 μm or more to 0.5 μm or less. Where thethickness of the antifouling composition 15 is less than 0.05 thedesired dust adhesion inhibiting effect sometimes cannot be obtained.Meanwhile where the thickness of the antifouling composition 15 is morethan 0.5 μm, defects such as cracks and voids easily appear in the film,depressions and protrusions where the contamination can be easilytrapped are formed on the surface, and the desired dust adhesioninhibiting effect sometimes cannot be obtained.

It is preferred that the water-based coating composition include thefluororesin particles 12 as the resin component, but admixing a resinsuch as included in a coating based on typical resin components is notdesirable. One of the features of the antifouling coating 15 inaccordance with the present invention is that dust or the like isunlikely to adhere to the porous silica film formed by cohesion of theultrafine silica particles 11. Therefore, where a resin component suchthat fills the gaps between the ultrafine silica particles 11 or a resincomponent such that the ultrafine silica particles 11 are completelycovered with the film is present in the water-based coating composition,this feature is lost and a high antifouling effect of the presentinvention cannot be obtained. The problem that the antifouling effectcannot be obtained does not arise when a particulate resin componentwhich does not cover the ultrafine silica particles 11 entirely with afilm is used.

The content of the resin component, or a component that reacts to becomea resin component, which can be added to the water-based coatingcomposition is preferably equal to or less than 50% by mass, morepreferably equal to or less than 30% by mass of the total amount of theultrafine silica particles 11, zirconium compound, and fluororesinparticles 12 (when such are included). The mass of the ultrafine silicaparticles 11 and the fluororesin particles 12 in this case is measuredby drying the water-based coating composition at 120° C. and removingthe moisture. The content of the zirconium compound is calculated on thebasis of the mass of zirconium oxide when the entire zirconium in theadded zirconium compound becomes zirconium oxide (ZrO₂). A water-solubleresin component is included in the content of the abovementioned resincomponent. Even when the water-soluble resin component is added to thewater-based coating composition, this component is easily removed bycontact between the antifouling coating 15 and water, thereby ensuringantifouling. Examples of the water-soluble resin component includesurfactants, dispersants, and flocculants.

A method for coating the water-based coating composition is notparticularly limited, and the coating can be performed using a brush, asprayer, or a roll coater. In the case of brush or spray coating, whenthe excess water-based coating composition remaining on the surface isdried, thick portions are locally formed and a problem such as increaseof the adhesion of dust at those portions may arise. Therefore, it ispreferred that the extra water-based coating composition be removed byallowing the coated solution to stay in order to eliminate the extrawater-based coating composition, or by blowing off the extra water-basedcoating composition with an air blower.

Heating is performed, as necessary, after the antifouling coating 15 hasbeen formed. Where the heating is performed, it is preferred that thecoating be allowed to stay in an oven at a temperature from 40° C. ormore to 150° C. or less or that hot air at a temperature from 40° C. ormore to 150° C. or less be blown. Where the temperature is less than 40°C., the heating effect is not demonstrated. Meanwhile, it is undesirablethat the temperature be higher than 150° C., because such a hightemperature is highly probable to degrade the hydrophilicity of theantifouling coating 15. The heating time is preferably 15 sec or more to15 min or less. It is undesirable that the heating time be less than 15sec, because the temperature of the fin material often does not risesufficiently. Where the heating time exceeds 15 min, the productivity islost and the decrease in hydrophilicity is advanced.

Embodiment 2

The antifouling coating according to Embodiment 2 of the presentinvention has a hydrophilic organic coating as a primary layer for aninorganic coating. Where the antifouling coating of such a form is usedin a heat exchanger, good heat exchanger characteristics are obtained.As a result of forming a hydrophilic organic coating as a primary layeron the surface of a heat exchanger, for example, the surface of fins,forming a reaction layer from an aqueous solution comprising a zirconiumcompound which is at least one selected from zirconium chloride andzirconyl chloride on the hydrophilic organic coating, and forming aninorganic coating from a water-based coating composition comprisingultrafine silica particles on the reaction layer, or forming aninorganic coating from a water-based coating composition comprisingultrafine silica particles and a zirconium compound which is at leastone selected from zirconium chloride and zirconyl chloride, it ispossible to inhibit the corrosion of fins and maintain thehydrophilicity and antifouling ability for a long time. The hydrophilicorganic coating demonstrates an effect of inhibiting the corrosion ofthe heat exchanger surface. Since the inorganic coating increases thehydrophilicity of the surface, the condensed water is prevented fromforming liquid droplets and increasing the ventilation resistance, andthe adhesion of dust and fibrous contaminants sucked into the heatexchanger is suppressed.

FIG. 3 is a schematic diagram illustrating the degradation of theconventional coating caused by adhesion of metal particles. In theconventional coating depicted in FIG. 3, an inorganic coating 23 inwhich ultrafine silica particles 11 are aggregated and fluororesinparticles 12 are distributed in a spot-like fashion is laminated on ahydrophilic organic coating 22 constituted by a polymer 21. With thecoating of such a configuration, as has been explained with reference toFIG. 1, dust is unlikely to adhere, but when the coating is wetted withwater, the metal particle 13 (for example, an iron particle) adheresthereto. As a result of the metal particle 13 being partially corrodedby water, metal ions 14 are released from the metal particle 13. Thereleased metal ions 14 penetrate through the inorganic coating 23 andreach the hydrophilic organic coating 22. The metal ions 14 function asa catalyst and enhance the decomposition of the polymer constituting thehydrophilic organic coating 22. As a result, the hydrophilic organiccoating 22 is degraded, the inorganic coating 23 is peeled off, anddefects appear in the entire coating. As a result, not only is the metalparticle 13 attached to the coating, but where the base material is froma metal, the corrosion thereof is advanced. In such a state, thehydrophilicity and dust adhesion inhibiting effect of the inorganiccoating 23 are degraded.

Meanwhile, in the antifouling coating 15 according to Embodiment 2 ofthe present invention, the effect of the metal particle 13 can beinhibited by using a coating composition comprising a zirconium compoundwhich is at least one selected from zirconium chloride and zirconylchloride when forming the inorganic coating 23 on the hydrophilicorganic coating 22. FIG. 4 is a schematic diagram demonstrating that theantifouling coating 15 according to Embodiment 2 of the presentinvention is unlikely to be degraded by the adhesion of the metalparticle 13. Zirconium chloride or zirconyl chloride act upon theinorganic coating 23 in the same manner as explained in Embodiment 1,but also act to inhibit the effect of the metal particle 13 on thehydrophilic organic coating 22. Thus, zirconium chloride or zirconylchloride act upon the hydrophilic organic coating 22 when an aqueoussolution comprising a zirconium compound which is at least one selectedfrom zirconium chloride and zirconyl chloride is coated on thehydrophilic organic coating 22 and also when a water-based coatingcomposition comprising the ultrafine silica particles, a zirconiumcompound which is at least one selected from zirconium chloride andzirconyl chloride, and water, which is described in Embodiment 1, iscoated on the hydrophilic organic coating 22. The Zr atoms 16 introducedby the zirconium chloride or zirconyl chloride react with the hydroxylgroups in the hydrophilic organic coating 22, crosslink the polymercomponents, and strengthen the hydrophilic organic coating 22. Such anaction can inhibit the effect of the metal particle 13. FIG. 4illustrates a state in which the hydrophobic fluororesin particles 12are present in a spot-like fashion in the thin hydrophilic silica film,but the fluororesin particles 12 further inhibit the adhesion of dust,and even when such particles are not present, the effect of inhibitingthe attachment of the metal particle 13 can be obtained as a result ofadding zirconium chloride or zirconyl chloride. Thus, since the surfaceof such antifouling coating 15 in such heat exchanger is constituted bya continuous hydrophilic portion in which hydrophobic portions arepresent, when water droplets adhere thereto, the droplets are easilyspread and the film, when viewed as a whole, demonstrates highhydrophilicity. Further, because of a configuration in which hydrophilicportions and hydrophobic portions are finely mixed, even whenhydrophilic dust such as sand dust or hydrophobic dust, such as soot,flies together with the air flow and attempts to adhere to the heatexchanger surface, the dust comes into contact with the surface havingopposite nature and cannot stably adhere to the surface. Thus, strongantifouling ability is demonstrated with respect to a wide range ofdust-like contaminants.

The hydrophilic organic coating 22 of Embodiment 2 may be a film thatincludes a polymer having a polar group and that is not soluble inwater. The type of the polymer is not particularly limited, and examplesof suitable polymers include polyvinyl alcohol, polyvinyl pyrrolidone,and polyacrylamide homopolymers, copolymers and modification productsthereof, acrylic acid and methacrylic acid homopolymers, copolymers andsalts thereof, and various epoxy resins and urethane resins. Thosecompounds may be used individually or as mixtures of different typesthereof. Further, among fluororesins and silicones, those having polargroups can be used. Further, a composition in which polymers of a largenumber of types are mixed homogeneously or exist as separate phases,have a particulate shape, or are mixed with a component functioning as abinder therefor may be also used.

A crosslinking agent, a radical initiator, a reactive component, andinorganic particles such as silica and titania may be also added to thecoating composition for forming the hydrophilic organic coating 22 inorder to make it insoluble or increase the strength thereof.

The thickness of the hydrophilic organic coating 22 is preferably from0.1 μm or more to 15 μm or less. Where the thickness of the hydrophilicorganic coating 22 is less than 0.1 μm, the coating will be too thin,and sufficient anticorrosive effect sometimes cannot be obtained.Meanwhile, it is also undesirable that the thickness of the hydrophilicorganic coating 22 exceed 15 μm, because the film becomes too thick andthe efficiency of heat transfer decreases.

The reaction of zirconium chloride or zirconyl chloride may be performedwith respect to the hydrophilic organic coating 22, the inorganiccoating 23, or both coatings.

Where the zirconium chloride or zirconyl chloride is caused to actdirectly on the hydrophilic organic coating 22, an aqueous solutioncomprising zirconium chloride or zirconyl chloride may be coated on thehydrophilic organic coating 22. With this method, a reaction layer isformed by reacting the zirconium chloride or zirconyl chloride at asufficiently high density with the hydrophilic organic coating 22,instead of relying on a complex process. The advantage of such areaction layer is that metal ions 14 released from the metal particle 13can be more effectively prevented from penetrating into the hydrophilicorganic coating 22. Furthermore, as explained in Embodiment 1, theconcentration of zirconium chloride or zirconyl chloride in thewater-based coating composition comprising ultrafine silica particles11, a zirconium compound which is at least one selected from zirconiumchloride and zirconyl chloride, and water has a low value equal to orless than 50% by mass of the solid weight of the ultrafine silicaparticles, but with the method of coating an aqueous solution comprisingzirconium chloride or zirconyl chloride, a higher concentration can beobtained. Therefore, zirconium chloride or zirconyl chloride can bereacted with the hydrophilic organic coating 22 to obtain a higherdensity. In the aqueous solution to be used in the case of a directreaction with the hydrophilic organic coating 22, a zirconium compoundwhich is at least one selected from zirconium chloride and zirconylchloride is contained preferably at 0.1% by mass or more to 40% by massor less, more preferably at 0.2% by mass or more to 15% by mass or less.Where the content of the zirconium compound is less than 0.1% by mass,the number of Zr atoms 16 introduced into the hydrophilic organiccoating 22 is too small and a sufficient effect sometimes cannot beobtained. Meanwhile, where the content of the zirconium compound exceeds40% by mass, excess compound adheres to the surface and they canadversely affect the formation of the inorganic coating 23. The contentof the zirconium compound, as referred to herein, is calculated on thebasis of the mass of zirconium oxide when the entire zirconium in theadded zirconium compound becomes zirconium oxide (ZrO₂).

A method for coating the abovementioned aqueous solution is notparticularly limited and the coating may be performed by using a sprayeror a roller, or by dipping or casting. The coating can be dried bynatural drying at a normal temperature, but the reaction can beaccelerated and a stronger hydrophilic organic coating 22 can beobtained by performing the drying with hot air or by heating in an oven.The heating temperature in this case is preferably from 40° C. or moreto 250° C. or less. Where the heating temperature is less than 40, thedrying is not performed rapidly. It is also undesirable that the heatingtemperature exceed 250° C. because the hydrophilic organic coating 22can be thermally degraded and cracks can appear therein. The merit ofthe heating treatment performed at this time is that where the inorganiccoating 23 is treated, the treatment can be performed at a temperatureat which a problem such as decrease in hydrophilicity arises.

Further, as a result of forming the inorganic coating 23 on the reactionlayer by coating a water-based coating composition comprising theultrafine silica particles 11, an antifouling coating 15 is obtainedwhich is constituted of the hydrophilic organic coating 22, the reactionlayer, and the inorganic coating 23. The type and content of theultrafine silica particles 11 used in this case are the same as inEmbodiment 1. Further, the fluororesin particles 12 may be alsointroduced into the water-based coating composition used herein, and thetype and content thereof are the same as in Embodiment 1.

A method for coating the water-based coating composition is notparticularly limited, and the coating can be performed using a brush, asprayer, or a roll coater. In the case of brush or spray coating, whenthe excess water-based coating composition remaining on the surface isdried, thick portions are locally formed and a problem such as increaseof the adhesion of dust at those portions may arise. Therefore, it ispreferred that the extra water-based coating composition be removed byallowing the coated solution to stay in order to eliminate the extrawater-based coating composition, or by blowing off the extra water-basedcoating composition with an air blower.

Heating is performed, as necessary, after the inorganic coating 23 hasbeen formed. When said heating is performed, it is preferred that thecoating be allowed to stay in an oven at a temperature from 40° C. ormore to 150° C. or less or have hot air blowing on it at a temperaturefrom 40° C. or more to 150° C. or less. Where the temperature is lessthan 40° C., there will be no heating effect. Meanwhile, it is alsoundesirable that the temperature be higher than 150° C., because it ishighly probable that such a high temperature will degrade thehydrophilicity of the inorganic coating 23. The heating time ispreferably 15 sec or more to 15 min or less. It is undesirable that theheating time be less than 15 sec, because the temperature of the finmaterial often does not rise sufficiently. Where the heating timeexceeds 15 min, productivity is lost and the decrease in hydrophilicityis advanced.

Meanwhile, in order to cause the zirconium chloride or zirconyl chlorideto act upon the hydrophilic organic coating 22 and the inorganic coating23, a method can be used by which the hydrophilic organic coating 22 andthe inorganic coating 23 are formed and then treated with an aqueoussolution comprising a zirconium compound which is at least one selectedfrom zirconium chloride and zirconyl chloride. A method that involvessimpler steps than the above-described method includes coating awater-based coating composition comprising the ultrafine silicaparticles 11, a zirconium compound which is at least one selected fromzirconium chloride and zirconyl chloride, and water on the hydrophilicorganic coating 22, this method being described in Embodiment 1. Withthis method, the step of coating an aqueous solution including azirconium compound which is at least one selected from zirconiumchloride and zirconyl chloride can be omitted, and the adhesion of thehydrophilic organic coating 22 and the inorganic coating 23 can beincreased.

A method by which the contact time of the water based coatingcomposition and the hydrophilic organic coating 22 prior to drying isextended, for example, a method by which the time of dipping into thewater-based coating composition or the time of spraying the water-basedcoating composition with a sprayer is extended, can be used to increasethe reliability of the reaction of zirconium chloride or zirconylchloride. With this method, the reaction can be sufficiently enhanced bysetting the contact time to 10 sec or longer, preferably to about 30sec. Where the contact time is less than 10 sec, no difference with theusual coating can be found. It is also undesirable that the contact timebe in excess of 30 sec because practically no additional effect isproduced. Increasing the temperature of the water-based coatingcomposition or coating object is another method for making the reactionof zirconium chloride or zirconyl chloride more reliable. The reactionis easily accelerated by setting this temperature from 30° C. or more to60° C. or less. Where the temperature is less than 30° C., the heatingeffect is too small. It is also undesirable that the temperature behigher than 60° C. because the evaporation of the water-based coatingcomposition is intensified and drying is accelerated, yet conversely,the reaction with the hydrophilic organic coating 22 is accordinglydecelerated and thus a homogeneous coating is difficult to obtain.

Where the above-described antifouling coating 15 is formed on the heatexchanger fins, it is preferred that the coating composition be appliedto the fins after the pipe in which the coolant flows has been joined tothe fins and the heat exchanger has been assembled. Where the heatexchanger is assembled after forming the antifouling coating 15 on thefins, since the fin material is subjected to such operations aspunching, pressing, and pipe insertion, the antifouling coating 15 canbe damaged. This can be avoided by applying the coating composition tothe fins after the pipe in which the coolant flows has been joined tothe fins and the heat exchanger be assembled. The coating in this casecan be performed by spraying or dipping. It is also preferred thatexcess liquid be removed by allowing the coating composition or aqueoussolution to stay after the coating application to eliminate the excessliquid, or by shaking off the excess liquid by moving, for example,rotating, the heat exchanger, or by blowing the extra liquid off with anair blower. It is also preferred that only the formation of thehydrophilic organic coating 22 be performed on the unassembled finmaterials and that the aqueous solution or water-based coatingcomposition be coated after the heat exchanger has been assembled.

EXAMPLES

The present invention will be explained in detail below with referenceto examples, but the present invention is not limited to these examples.

Example 1

A water-based coating composition having the composition shown in Table1 was prepared by stirring and mixing pure water, colloidal silica(Snowtex OXS manufactured by Nissan Kagaku Kogyo KK) comprisingultrafine silica particles with an average particle size of 6 nm, a PTFEdispersion with an average particle size of 150 nm, and zirconiumchloride (12% by mass of the ultrafine silica particles), adding anonionic surfactant (polyoxyethylene lauryl alkyl ester) at 0.1% by massto the mixture, and further stirring and mixing.

The obtained water-based coating composition was coated on an aluminumfin material and dried with an air blower. The aluminum fin materialwith the coating formed thereon was wetted by dipping it into water, aniron powder with an average particle size of 45 μm was blown thereon,followed by drying with an air blower, and the adhesion state of theiron powder was observed. The adhesion state of the iron powder wasvisually evaluated. The evaluation used 6 grades. When the iron powderadhesion was induced to the untreated aluminum fin (Comparative Example4), a large amount of iron powdered has adhered, and this state wasevaluated by grade 5, the state in which absolutely no iron powder hasadhered was evaluated by grade 0.

The aluminum fin material with the iron powder adhered thereto wasallowed to stay for 3 days at a humidity of 90% and then sprayed withwater. The contamination state thereof was then visually checked.

The contact angle was measured with a contact angle meter PD-X (KyowaKaimen Kagaku).

The results are shown in Table 2.

Example 2

A water-based coating composition having the composition shown in Table1 was prepared by stirring and mixing pure water, colloidal silica(Snowtex OXS manufactured by Nissan Kagaku Kogyo KK) comprisingultrafine silica particles with an average particle size of 6 nm, a PTFEdispersion with an average particle size of 150 nm, and zirconiumchloride (10% by mass of the ultrafine silica particles), adding anonionic surfactant (polyoxyethylene lauryl alkyl ester) at 0.1% by massto the mixture, and further stirring and mixing. An aluminum finmaterial with a coating formed thereon was fabricated in the same manneras in Example 1, except that this water-based coating composition wasused. The evaluation was performed in the same manner as in Example 1.The results are shown in Table 2.

Example 3

An aluminum fin material with a coating formed thereon was fabricated inthe same manner as in Example 1, except that zirconyl chloride was usedinstead of zirconium chloride. The evaluation was performed in the samemanner as in Example 1. The results are shown in Table 2.

Example 4

An aluminum fin material with a coating formed thereon was fabricated inthe same manner as in Example 2, except that zirconyl chloride was usedinstead of zirconium chloride. The evaluation was performed in the samemanner as in Example 1. The results are shown in Table 2.

Example 5

An aluminum fin material with a coating formed thereon was fabricated inthe same manner as in Example 1, except that the PTFE dispersion was notused. The evaluation was performed in the same manner as in Example 1.

The results are shown in Table 2.

Comparative Example 1

An aluminum fin material with a coating formed thereon was fabricated inthe same manner as in Example 1, except that the content of zirconiumchloride was changed to 60% by mass of the ultrafine silica particles.The evaluation was performed in the same manner as in Example 1. Theresults are shown in Table 2.

Comparative Example 2

An aluminum fin material with a coating formed thereon was fabricated inthe same manner as in Example 1, except that zirconium chloride was notused. The evaluation was performed in the same manner as in Example 1.The results are shown in Table 2.

Comparative Example 3

An aluminum fin material with a coating formed thereon was fabricated inthe same manner as in Example 1, except that the PTFE dispersion andzirconium chloride were not used. The evaluation was performed in thesame manner as in Example 1. The results are shown in Table 2.

Comparative Example 4

The untreated aluminum fin material (the aluminum fin material on whichthe coating has not been formed) was evaluated in the same manner as inExample 1. The results are shown in Table 2.

TABLE 1 Ultrafine silica Fluororesin Zirconium particles particlescompound (% by mass) (% by mass) (% by mass) Example 1 2.5 0.5 0.3(zirconium chloride) Example 2 3.0 1.0 0.3 (zirconium chloride) Example3 2.5 0.5 0.3 (zirconyl chloride) Example 4 3.0 1.0 0.5 (zirconylchloride) Example 5 2.5 None 0.3 (zirconium chloride) Comparative 2.50.5 1.5 (zirconium Example 1 chloride) Comparative 2.5 0.5 None Example2 Comparative 2.5 None None Example 3 Comparative None None None Example4

TABLE 2 State after wet Attachment state adhesion of of iron powderafter Contact iron powder stationary period and angle and drying washingwith water Example 1 11° 1 Iron powder slightly attached Example 2 12° 2Iron powder slightly attached Example 3 11° 2 Iron powder not attachedExample 4 12° 1 Iron powder slightly attached Example 5 11° 3 Ironpowder not attached Comparative 25° 4 Iron powder not Example 1 attachedComparative 11° 2 Iron powder attached Example 2 Comparative 12° 4 Ironpowder attached Example 3 Comparative 45° 5 Iron powder attached Example4

The results shown in Table 2 that relate to the adhesion of iron powderdemonstrate that the adhesion to the aluminum fin material coated withthe ultrafine silica film (Comparative Example 3) decreased with respectto that to the untreated aluminum fin material of Comparative Example 4,and the adhesion to the aluminum fin material coated with the ultrafinesilica film and fluororesin particles (Comparative Example 2) decreasedfurther. With the coating comprising excess zirconium chloride(Comparative Example 1), the iron powder adhesion inhibition effectdecreased. The hydrophilicity was increased by the coating in allexamples, except for Comparative Example 1 with excess zirconiumchloride in which the hydrophilicity decreased.

High hydrophilicity was obtained in all of Examples 1 to 5, and theadhesion of iron powder in those examples was small. In Example 5, theadhesion of iron powder was slightly larger than in Examples 1 to 4,which is due to the fact that the fluororesin particles were not added.Further, in Example 5, the adhesion of iron powder was less than inComparative Example 3. This result indicates that the addition ofzirconium chloride inhibits the adhesion of iron powder.

When the iron powder was corroded, in Comparative Examples 2 to 4 inwhich no zirconium compound was added, the rust strongly attached to thesurface and was not stripped with the sprayer, whereas in Examples 1 to5 and Comparative Example 1, the rust attachment was significantlyinhibited. Further, in Examples 1 to 5, high hydrophilicity and dustadhesion inhibiting effect were obtained and it was difficult for theadhered iron powder to attach even after being corroded.

Examples 6 and 7

An aqueous solution in which 3% by mass of polyvinyl alcohol Z-200(Nippon Gosei Kagaku KK) was mixed with 5% by mass, on the basis of thepolyvinyl alcohol, of glyoxazole was coated on an aluminum fin material,and heated for 5 min at 150° C. to form a hydrophilic organic coatingwith a thickness of 0.7 μm. An aqueous solution comprising 5% by mass ofzirconyl chloride was coated on the hydrophilic organic coating anddried with an air blower at a normal temperature to form a reactionlayer (pretreatment). Then, a water-based coating composition comprisingthe components presented in Table 3 was coated on the reaction layer anddried with an air blower to form an inorganic coating.

The aluminum fin material after the formation of the coating was wettedby dipping it into water, an iron powder with an average particle sizeof 45 μm was blown thereon, drying was performed with an air blower, andthe adhesion state of the iron powder was observed. The adhesion stateof the iron powder was determined visually. The evaluation used 6grades. When only the hydrophilic organic coating was present(Comparative Example 5), a black stained state was observed and thisstate was evaluated as grade 5, and the state in which absolutely noiron powder adhered was evaluated as grade 0.

The coating was allowed to stay for 3 days at a humidity of 90% in astate with adhered iron powder, water was then sprayed with a sprayer,and the state of the corroded iron powder was checked. Changes in thecoating close to the iron particles were checked by immersing thecoating for 1 min into a 1% aqueous solution of sodium hydroxide. Theresults are shown in Table 4.

Examples 8 and 9

An aqueous solution in which 3% by mass of polyvinyl alcohol Z-200(Nippon Gosei Kagaku KK) was mixed with 5% by mass, on the basis of thepolyvinyl alcohol, of glyoxazole was coated on an aluminum fin material,and heated for 5 min at 150° C. to form a hydrophilic organic coatingwith a thickness of 0.7 μm. A water-based coating composition comprisingthe components presented in Table 3 was coated on a reaction layer anddried with an air blower to form an inorganic coating on the hydrophilicorganic coating, thereby producing an aluminum fin material with acoating formed thereon. The evaluation was performed in the same manneras in Examples 6 and 7. The results are shown in Table 4.

Comparative Example 5

An aqueous solution in which 3% by mass of polyvinyl alcohol Z-200(Nippon Gosei Kagaku KK) was mixed with 5% by mass, on the basis of thepolyvinyl alcohol, of glyoxazole was coated on an aluminum fin material,and heated for 5 min at 150° C. to form a hydrophilic organic coatingwith a thickness of 0.7 μm, thereby producing an aluminum fin materialwith a coating formed thereon. The evaluation was performed in the samemanner as in Examples 6 and 7. The results are shown in Table 4.

Comparative Examples 6 and 7

An aqueous solution in which 3% by mass of polyvinyl alcohol Z-200(Nippon Gosei Kagaku KK) was mixed with 5% by mass, on the basis of thepolyvinyl alcohol, of glyoxazole was coated on an aluminum fin material,and heated for 5 min at 150° C. to form a hydrophilic organic coatingwith a thickness of 0.7 μm. A water-based coating composition comprisingthe components presented in Table 3 was coated on a reaction layer anddried with an air blower to form an inorganic coating on the hydrophilicorganic coating, thereby producing an aluminum fin material with acoating formed thereon. The evaluation was performed in the same manneras in Examples 6 and 7. The results are shown in Table 4.

TABLE 3 Ultrafine silica Fluororesin Zirconyl particles particleschloride (% by mass) (% by mass) (% by mass) Example 6 1.5 NonePretreatment Example 7 1.5 0.3 Pretreatment Example 8 1.5 None 0.3Example 9 1.5 0.3 0.5 Comparative — — — Example 5 Comparative 1.5 NoneNone Example 6 Comparative 1.5 0.3 None Example 7

TABLE 4 State after wet Attachment state Film state of iron adhesion ofof iron powder after powder adhesion Contact iron powder stationaryperiod and portion after angle and drying washing with water washingwith alkali Example 6  9° 2 Iron powder not No peeling attached, nocoloration Example 7 10° 1 Iron powder slightly No peeling attached, nocoloration Example 8 12° 2 Iron powder not No peeling attached, slightcoloration Example 9 12° 1 Iron powder not No peeling attached, slightcoloration Comparative 45° 5 Iron powder attached, Peeling Example 5coloration Comparative 11° 1 Iron powder attached, Peeling Example 6coloration Comparative 10° 2 Iron powder attached, Partial peelingExample 7 coloration

The hydrophilicity was determined from the contact angle, theantifouling performance is determined from the state of the iron powderafter wet adhesion and drying, the iron rust attachment state andcoloration caused by iron ions are determined from the attachment stateof the iron powder after washing with water, and the degree ofdegradation of the hydrophilic organic coating caused by iron ions isdetermined from the state of the coating in the iron powder adhesionportions after washing with an alkali.

According to the results in Table 4, in Comparative Example 5, thecontact angle in a dry state is 45° and the hydrophilicity is low. InComparative Example 5, hydrophilicity increases in the humidified stateand the coating becomes hydrophilic. In Comparative Example 5, the ironpowder easily adhered and the antifouling ability is low. In ComparativeExample 5, when the coating is stored in a humidified state, iron rustis attached thereto and the coating is discolored into a brown-blackcolor. This result indicates that the attachment of dust comprising theiron powder, or the like, easily occurs. Further, in alkali washing, thecoating has dissolved and peeled off in the iron powder adhesionportions. It follows from this result that the film has been degraded bythe produced iron ions.

In Comparative Examples 6 and 7, the hydrophilic organic coating iscovered by the inorganic coating. The contact angle indicates that thehydrophilicity is high, and the iron powder adhesion state indicatesthat the antifouling ability is also high. However, where the coating isstored in a humidified state, iron rust is easily attached, and thehydrophilic organic coating is degraded and peeled off.

In Examples 6 and 7, the hydrophilicity is high and the antifoulingperformance is also high. Even when the iron powder is corroded, thelevel of attachment and coloration is low. Furthermore, the hydrophilicorganic coating is not peeled off by an alkali and the effect of ironions is inhibited. In particular, the coloration of the hydrophilicorganic coating is significantly suppressed, and the polymer componentsare apparently efficiently protected by direct treatment of thehydrophilic organic coating. The comparison of Example 6 and Example 7demonstrates that Example 7 is superior in the antifouling performance,and the effect of the addition of fluororesin particles is demonstrated.

In Examples 8 and 9, the hydrophilicity is high and the antifoulingperformance is also high. Even when the iron powder is corroded, thelevel of attachment and coloration is low. Furthermore, the hydrophilicorganic coating is not peeled off by an alkali and the effect of ironions is inhibited. In particular, the attachment of iron rust issignificantly suppressed, and the inorganic coating is apparentlyefficiently protected due to the addition of zirconium chloride to theinorganic coating. The comparison of Example 8 and Example 9demonstrates that Example 9 is superior in the antifouling performance,and the effect of the addition of fluororesin particles is demonstrated.

EXPLANATION ON NUMERALS

11—ultrafine silica particles; 12—fluororesin particles; 13—metalparticle; 14—metal ions; 15—antifouling coating; 16—Zr atoms;21—polymer; 22—hydrophilic organic coating; 23—inorganic coating.

The invention claimed is:
 1. A method for manufacturing a heatexchanger, comprising: joining a fin with a hydrophilic organic coatingformed thereon to a pipe in which a coolant flows; and forming, on thehydrophilic organic coating, a reaction layer from an aqueous solutioncomprising a zirconium compound which is at least one selected fromzirconium chloride and zirconyl chloride and then forming, on thereaction layer, an inorganic coating from a water-based coatingcomposition comprising 0.1% by mass to 10% by mass of ultrafine silicaparticles having an average particle size equal to or less than 25 nm.2. A method for manufacturing a heat exchanger according to claim 1,wherein the water-based coating composition further comprisesfluororesin particles having an average particle size from 50 nm to 500nm.
 3. A heat exchanger comprising a pipe in which a coolant flows, anda fin attached to the pipe, the heat exchanger comprising: (a) ahydrophilic organic coating formed on the fin; a reaction layer obtainedby reacting a zirconium compound which is at least one selected fromzirconium chloride and zirconyl chloride on a surface layer of thehydrophilic organic coating; and an inorganic coating provided on thereaction layer and formed from a water-based coating compositioncomprising 0.1% by mass to 10% by mass of ultrafine silica particleshaving an average particle size equal to or less than 25 nm.
 4. Themethod of claim 1, wherein the water-based coating composition comprises0.2% by mass to 10% by mass of ultrafine silica particles having anaverage particle size equal to or less than 25 nm.
 5. The method ofclaim 1, wherein the water-based coating composition comprises 0.2% bymass to 4% by mass of ultrafine silica particles having an averageparticle size equal to or less than 25 nm.
 6. The method of claim 1,wherein the zirconium compound is zirconium chloride and zirconylchloride.
 7. The method of claim 1, wherein the zirconium compound iszirconium chloride.
 8. The method of claim 1, wherein the zirconiumcompound is zirconyl chloride.
 9. The heat exchanger of claim 3, whereinthe water-based coating composition comprises 0.2% by mass to 10% bymass of ultrafine silica particles having an average particle size equalto or less than 25 nm.
 10. The heat exchanger of claim 3, wherein thewater-based coating composition comprises 0.2% by mass to 4% by mass ofultrafine silica particles having an average particle size equal to orless than 25 nm.
 11. The heat exchanger of claim 3, wherein thezirconium compound is zirconium chloride and zirconyl chloride.
 12. Theheat exchanger of claim 3, wherein the zirconium compound is zirconiumchloride.
 13. The heat exchanger of claim 3, wherein the zirconiumcompound is zirconyl chloride.